Solid-state secondary lithium ion batteries are constructed from polymeric electrodes bonded to current collectors and separated by a polymeric separator.
The anode of a lithium ion battery may be constructed from a carbonaceous material. These carbonaceous materials can reversibly accept and donate significant amounts of lithium. Examples of suitable carbonaceous materials include synthetic and natural graphite, petroleum coke and doped coke. The anode may be constructed from transition metal compounds having layered structures into which lithium ions can be intercalated and deintercalated during charge and discharge. These cells are referred to as rocking chair cells. The rocking chair compounds have electrochemical potentials close to that of lithiated carbon and different from the transition metal oxides frequently used for cathodes. Examples of suitable rocking chair anode compounds are Li.sub.x WO.sub.2, Li.sub.x MoO.sub.2 and Li.sub.x TiS.sub.2.
Transition metal oxides are preferred for lithium ion battery cathodes. Li ions can be inserted into and extracted from these intercalating compounds with little or no structural modification of the compound. Examples of suitable transition metal oxides for cathode construction include LiCoO.sub.2, LiNiO.sub.2 and LiMn.sub.2 O.sub.4.
Anode and cathode active materials are often formed into anode and cathode structures by binding the active materials in a polymer film or sheet. U.S. Pat. Nos. 4,980,250, 5,219,680, 5,340,670, 5,380,606, 5,426,006, 5,582,931, 5,584,893, 5,643,695 and 5,656,393, incorporated herein by reference, disclose compositions for Li ion cathodes and anodes and methods of making these electrodes. Once the polymer films containing the electrode active materials have set, the electrodes, separator and collectors are generally laminated by heat and pressure.
The separator must provide sufficient insulation between electrodes to prevent the formation of an electrical circuit which causes a short, while at the same time being permeable to migrating lithium ions. Separators have been constructed of very thin sheets of polymer. The polymer matrix may be composed of olefin, polyvinyl alcohol, polyvinylidene difluoride and associated copolymers and the like. Alternatively, a preformed porous woven or nonwoven polymer mat may be utilized as a separator.
The electrolyte of lithium ion batteries consists of a lithium salt in a nonaqueous solvent. Typical lithium salts include LiPF.sub.6, LiASF.sub.6, LiBF.sub.4, LiClO.sub.4, LiN(CF.sub.3 SO.sub.2).sub.3 and LiN(SO.sub.2 C.sub.2 F.sub.5).sub.3. A nonaqueous environment is maintained in Li ion batteries because lithium and its salts are notoriously reactive in aqueous solutions. Aprotic organic solvents such as propylene carbonate or ethylene carbonate are commonly used. Lithium salts readily disperse in these solvents. Other solvents are tetrahydrofuran, 1,2-dimethoxyethane, dimethyl carbonate, diethyl carbonate, and diethoxyethane. The polymer separator, of course, must be stable to the solvent selected. For a discussion of conventional solvent/lithium solute systems, see S. Hossain, "Rechargeable Lithium Batteries (Ambient Temperature)", in Handbook of Batteries and Fuel Cells, D. Linden, Ed., McGraw-Hill, 2nd Ed., 1995, incorporated herein by reference.
An important aspect of the formation of separate or composite electrode and separator polymeric sheets is that the polymer be porous so that is may absorb the electrolyte and also to facilitate ion movement during charging and discharging. It is known in the art to utilize small, organic molecules such as plasticizers to render the polymer porous. For examples of patents disclosing the use of plasticizers in the formation of solid state lithium ion battery components see U.S. Pat. Nos. 5,418,091, 5,460,904, 5,540,741 and 5,552,239, incorporated herein by reference.
In general, the plasticizer is mixed with solvent, polymer, and other electrode or separator materials to form a slurry which is cast into a polymer sheet by means such as a Doctor blade. The slurry is allowed to cure, forming a polymer sheet. The separate electrode and separator polymer sheets are then laminated. Before further processing of the laminated structures, it is often desirable to remove the plasticizer to create a "dry" laminated structure.
Various methods of removing plasticizer are known in the art. U.S. Pat. Nos. 5,418,091 and 5,460,904 disclose the use of solvents such as diethyl ether to extract the plasticizer from the battery structures. In that method, the plasticizer containing battery components are washed with a compatible solvent which leaches the plasticizer without damaging or otherwise affecting the polymer. U.S. Pat. No. 5,552,239 discloses the use of supercritical fluid extraction to remove plasticizer. Supercritical fluid extraction utilizes an easily condensed gas such as carbon dioxide as a solvent at a temperature above its critical point.
These methods of plasticizer extraction have several inherent problems. Solvent extraction requires the use of expensive, flammable chemicals which may be damaging to the environment. In order to recycle the plasticizer, it must be distilled from the solvent. Supercritical fluid extraction has the advantage of low solvent cost and low toxicity. However, expensive equipment is required.
What is needed in the art is an apparatus and method for removing plasticizer not only from solid state secondary lithium ion batteries, but from other polymeric structures as well. The method should use low cost equipment, be performed in the absence of solvents, and allow recovery of the plasticizer in a form which can be readily and easily recycled.